A multiphase fluid is a fluid having more than one phase, such as a fluid having two or more liquid phases or a combination of a gas phase with one or more liquid phases.
Multiphase fluids are frequently encountered in industry and it is often necessary or desirable to have the ability to determine their properties. For example, in the oil and gas industry, multiphase fluids are very common and it is often important to know the respective proportions or flowrates of the various phases, which typically may include a gas phase, a water phase and an oil phase.
Unfortunately, however, determining the properties and real boundaries of a multiphase fluid can be difficult for a variety of reasons. For example, there are many different flow regimes which are possible for multiphase fluids, making modelling of multiphase flow difficult. These flow regimes manifest themselves in different phase patterns which depend upon the composition and flowrate of the multiphase fluid.
Multiphase fluids can exhibit a multitude of these phase patterns and are prone to slippage between the various phases. Where a multiphase fluid contains more than one liquid phase, separate slugs or plugs of the different liquid phases may be dispersed unevenly throughout the flow. Where a multiphase fluid contains a gas phase, the gas may take the form of small bubbles, large slugs or even a discrete layer of gas above the liquid phase or phases. These phenomena are due in part to the effects of viscosity, gravity, inertia and interfacial friction and affect the ability to obtain reliable and consistent measurements of the properties of multiphase fluids when single phase fluid measurement apparatus and techniques are used.
As a result, there are three conventional approaches to determining the properties of multiphase fluids. The first approach involves separating the multiphase fluid into its component phases and then measuring the component phases separately using single phase measurement methods and devices. The second approach involves creating a pseudo single phase fluid by mixing the multiphase fluid to produce a homogeneous mixture which is then measured using single phase measurement methods and devices. The third approach involves methods and devices which are specifically designed for use in determining the properties of multiphase fluids. Each of these approaches has its drawbacks.
The first approach requires that the flow of the multiphase fluid be interrupted and diverted so that the multiphase fluid can be sampled and then separated for measurement. Due to the effects of different flow regimes and phase patterns, there is also a risk that the sample of multiphase fluid that is taken will not be representative of the multiphase fluid generally.
The second approach requires that the multiphase fluid be homogeneous at the time of measurement so that it will behave like a single phase fluid. Due to the tendency of multiphase fluids to segregate into phases, it is difficult to ensure the homogeneity of the multiphase fluid, particularly if the mixing must be interrupted to facilitate measurement. The second approach is also relatively inefficient due to the energy required to mix the multiphase fluid, which can result in the multiphase fluid experiencing a significant pressure drop during measurement.
The third approach potentially offers the most reliable determination of the properties of a multiphase fluid, but methods and devices based on this approach tend to be quite complex in order to take into account the effects of the many possible flow regimes, phase patterns and slippage of phases.
Some of these methods and devices utilize conventional principles of fluid mechanics and involve the direct application of either or both of Bernoulli's equation and the momentum equation to determine the properties of a multiphase fluid. Bernoulli's equation is useful for determining mass flowrate as a function of pressure change experienced by a fluid in response to a change in its velocity, while the momentum equation is useful for determining mass flowrate as a function of force exerted by a fluid in response to a change in its direction of flow.
For example, some efforts have used Bernoulli's equation to determine the mass flowrate of a multiphase fluid by measuring pressure differentials between different locations in a conduit (U.S. Pat. No. 3,926,050 (Turner et al); U.S. Pat. No. 4,144,754 (Pitts et al); U.S. Pat. No. 4,261,196 (Scheid); U.S. Pat. No. 4,282,760 (Pitts et al); U.S. Pat. No. 4,417,474 (Elderton); U.S. Pat. No. 4,453,415 (Carter); U.S. Pat. No. 5,591,922 (Segeral et al); U.S. Pat. No. 5,608,170 (Atkinson et al); U.S. Pat. No. 5,641,915 (Ortiz et al) and PCT International Publication No. WO 95/33980 (Kolpak et al)).
Other efforts have focused upon the application of the momentum equation to determine the mass flowrate of a multiphase fluid by measuring reaction forces exhibited by multiphase fluids as they encounter bends in a conduit (U.S. Pat. No. 4,513,625 (Campman et al); U.S. Pat. No. 4,569,232 (Kim); U.S. Pat. No. 4,612,814 (Campman); U.S. Pat. No. 4,677,859 (Chinery)).
A multiphase fluid cannot, however, be characterized completely by its mass flowrate, which is the product of volumetric flowrate multiplied by the density of the fluid. For example, the determination of volumetric flowrate requires knowledge of both the mass flowrate and density of the multiphase fluid and determination of density requires knowledge of both the mass flowrate and the volumetric flowrate of the multiphase fluid.
Consequently, measurements of pressure differentials or reaction forces are typically supplemented with at least one other measurement so that the multiphase fluid can be characterized completely (U.S. Pat. No. 3,926,050 (Turner et al); U.S. Pat. No. 4,144,754 (Pitts et al); U.S. Pat. No. 4,261,196 (Scheid); U.S. Pat. No. 4,282,760 (Pitts et al); U.S. Pat. No. 4,417,474 (Elderton); U.S. Pat. No. 4,513,625 (Campman); U.S. Pat. No. 4,569,232 (Kim); U.S. Pat. No. 4,612,814 (Campman); U.S. Pat. No. 4,677,859 (Chinery); U.S. Pat. No. 5,036,712 (Lew); U.S. Pat. No. 5,591,922 (Segeral); U.S. Pat. No. 5,608,170 (Atkinson et al); PCT International Publication No. WO 95/33980 (Kolpak)). In some cases, two different pressure differential measurements are used to characterize the multiphase fluid flow (U.S. Pat. No. 3,926,050 (Turner); U.S. Pat. No. 4,417,474 (Elderton); U.S. Pat. No. 5,591,922 (Segeral et al)).
As a result, multiphase fluid flow characterization methods and devices which involve the application of fluid mechanics equations such as Bernoulli's equation or the momentum equation share for the most part several common disadvantages. First, more than one measurement of the multiphase fluid is necessary in order to characterize the multiphase fluid completely. Second, the data obtained through measurement must be input into the equations in order to determine values for the properties of the multiphase fluid, thus necessitating the use of relatively complex data processing equipment in conjunction with such methods and devices. Third, these methods and devices may be somewhat prone to unreliable results, since they are based upon two or more independent and discrete measurements each of which includes its own range of error. Finally, those methods and devices which rely upon the measurement of one or more pressure differentials tend to be relatively inefficient due to the flow disruption that must be created to provide a measurable pressure differential between two points and due to different phase patterns which may be exhibited at the two points.
There have as a result been some attempts in the art to develop methods and devices for characterizing multiphase fluids which are less dependent upon the direct application of fluid mechanics equations.
U.S. Pat. No. 4,300,399 (Kuijpers et al) describes a method for determining individual flowrates of phases of a multiphase fluid which involves measuring for a time interval a pressure differential between two locations in a pipe bend, calculating both an average value for the pressure differential and an RMS value for the pressure differential, and then comparing the average value and the RMS value with predetermined reference data in order to determine the individual flowrates of the phases.
U.S. Pat. No. 5,051,922 (Toral) describes a method for determining individual flowrates of phases of a multiphase fluid which involves measuring for a time interval one or more flow related characteristics of a multiphase fluid (such as absolute pressure, differential pressure or void fraction), deriving from the measurements a plurality of parameters, comparing each of the parameters with a corresponding calibration map which relates that parameter to a range of possible flowrates for the individual phases, and then determining a value for the individual flowrates which is uniquely related to all of the parameters.
The inventions described in Kuijpers and Toral have some similarities. First, both utilize the measurement of one or more selected flow characteristics as a function of time to derive a set of parameters that can be used to characterize the multiphase fluid flow. Second, both apply statistical analysis techniques to measured flow characteristics instead of fluid mechanics equations in order to characterize multiphase fluid flow. Third, both appear to facilitate the determination of composition of a multiphase fluid without first determining mass flowrate.
Neither Kuijpers nor Toral, however, address directly the problems resulting from varying flow regimes, phase patterns and slippage which are associated with multiphase fluids. In particular, neither Kuijpers nor Toral allow for discrepancies in characterization which may occur due to the orientation of the multiphase fluid flow relative to gravity, since they both teach the measurement of flow characteristics in only one orientation relative to gravity. Since multiphase fluids behave differently depending upon the effects of both flow geometry and of orientation relative to gravity, measurements taken in only one orientation omit important information relating to variations in phase pattern which can be obtained through measurements taken in more than one orientation relative to gravity.
Problems resulting from varying flow regimes, phase patterns and slippage in relation to multiphase fluids have to some extent been recognized and addressed in the prior art.
U.S. Pat. No. 4,417,474 (Elderton) teaches a densitometer in which separate differential pressure measurements are made in horizontal and vertical portions of a pipeline and the difference between the two measurements is used to determine the density or specific gravity of the multiphase fluid.
U.S. Pat. No. 5,608,170 (Atkinson) teaches a method for determining the composition and flowrate of a multiphase fluid by making separate differential pressure measurements in a first flow passage and a second flow passage, wherein the second flow passage has a "different geometry" than the first flow passage. The "different geometry" referred to in Atkinson is defined in terms of either the area of the flow passages or the direction of flow relative to gravity.
Both Elderton and Atkinson apply fluid mechanics equations to the differential pressure measurements in order to arrive at the characterization of the multiphase fluid flow.
There is therefore a need for a method and apparatus for use in characterizing multiphase fluid flow which is not entirely dependent upon the application of fluid mechanics equations and which addresses the issues of flow regime, phase pattern and slippage as they apply to multiphase fluids. There is also a need for such a method and apparatus which is relatively simple to construct and use and which does not require significant disruption of the multiphase fluid flow or result in the multiphase fluid experiencing significant pressure drop during measurement.